The present invention relates to breathalyzers, and breath alcohol interlock devices for preventing operation of vehicles and other machines by intoxicated persons.
In the 19th century, law enforcement officials dealt with the problem of alcohol abusers by imprisoning them until they were sober. In the 20th century, the advent of high-speed transportation and complex machinery gave high priority to alcohol testing and screening. Automobiles traveling at ninety feet per second on the freeway are unforgiving of drivers with alcohol impairment. The same is true for a 300-passenger aircraft guided by an alcohol-impaired pilot attempting to land under minimum-visibility conditions. There is very little margin for error. People who operate complex equipment with their judgment impaired by alcohol may not only be a danger to themselves, but impact the safety of others.
Until recently, the main application of alcohol testing was to traffic law enforcement. The intent was to identify people suspected of driving under the influence of alcohol and remove them from the road. After arrest, law enforcement officers gave the subject a chemical test to determine his blood alcohol level. Subjects were either released or incarcerated and prosecuted, depending on what alcohol levels were illegal as dictated by state law. Until the mid-1940""s, the primary means of measuring blood alcohol levels involved either blood or urine sample testing, both of which were time-consuming and expensive procedures. In the late 1940""s, alcohol breath testing replaced blood and urine sample testing as a means of screening subjects and producing evidentiary results for prosecution.
In the 1980""s, railroad, nuclear, Department of Defense, and maritime employees came under Federally-mandated testing requirements. In each case, new laws followed a major, alcohol-related disaster. In 1991, the United States Congress passed the Omnibus Transportation Employee Testing Act. This legislation mandated alcohol testing for transportation personnel involved in safety-sensitive jobs. This mandate included airline pilots and cabin attendants, truck drivers, railroad crews, and gas pipeline workers. The US Department of Transportation further defined unacceptable maximum alcohol levels.
Since the mid-1980s, infrared (IR) technology has been the primary means of breath alcohol testing in the United States. Current technology uses infrared measurement systems that are made more specific for alcohol by using several optical filters. Breath alcohol levels are measured this way by passing a narrow band of IR light, selected for its absorption by alcohol, through one side of a breath sample chamber and detecting emergent light on the other side. The alcohol concentration is then determined by using the well-known Lambert-Beers law, which defines the relationship between concentration and IR absorption. This IR technology has the advantage of making real-time measurements; however, it is particularly difficult and expensive to achieve specificity and accuracy at low breath alcohol concentration levels. Also, the IR detector output is nonlinear with respect to alcohol concentration and must be corrected by measurement circuits. A more favored technology uses electrochemical cells, also known as fuel cells.
The fuel cell effect was discovered in the early 1800""s when a British scientist immersed two platinum electrodes in sulfuric acid electrolyte and supplied hydrogen at one electrode and oxygen at the other. The resulting reaction created a current flow between the electrodes. There was no practical application of fuel cells at that time because of high cost and technological problems. In the 1960s, researchers at the University of Vienna demonstrated a fuel cell that was specific for alcohol. This evolved into the present-day cell used in all fuel cell-based breath alcohol measurement instruments. In its simplest form, the alcohol fuel cell consists of a porous, chemically inert layer coated on both sides with finely divided platinum (called platinum black). The porous layer is impregnated with an acidic electrolyte solution, and platinum wire electrical connections are attached to the platinum black surfaces, and this assembly is mounted in a plastic case having a gas inlet that allows a breath sample to be introduced as shown in FIG. 1.
The exact chemistry of the reaction that takes place in an alcohol fuel cell is open to some conjecture. Researchers assume that the reaction converts alcohol to acetic acid. In the process, this conversion produces two free electrons per molecule of alcohol. This reaction takes place on the upper surface of the fuel cell. H+ ions are freed in the process, and migrate to the lower surface of the cell, where they combine with atmospheric oxygen to form water, consuming one electron per H+ ion in the process. Thus, the upper surface has an excess of electrons, and the lower surface has a corresponding deficiency of electrons. When the two surfaces are connected electrically, a current flows through this external circuit to neutralize the charge. This current is a direct indication of the amount of alcohol consumed by the fuel cell. With appropriate signal processing, breath alcohol concentrations directly can be displayed. Commercial fuel cell instruments, introduced in the mid-1970s and initially suitable for non-evidential alcohol breath testing, were improved sufficiently by 1980 to be certified for evidential use by the US Department of Transportation, and by a number of state agencies and foreign governments. The fuel cell has established a reputation for specificity and linearity of response over the complete range of alcohol concentration expected in the breath. This range is from 5 to 900 ppm or its equivalent in other units of measurement.
When a precise volume of breath sample is quickly introduced into a fuel cell, the output current from the cell rises from zero to a peak, and then ultimately decays back to zero. The rate at which this happens is highly dependent on the loading across the sensor terminals. FIG. 2 illustrates this effect, for loadings of 100 and 300 ohms, and for a shorted condition (0 ohms). Traditional fuel cell measurement instruments of the prior art have load resistors of several hundred to one thousand ohms, and the height of the voltage peak across the resistor is used as the measure of alcohol content of the sample. Although this technique produces good linearity, significant time elapses before an acceptable measurement can be obtained, and the measurement cycles are objectionably long because complete conversion of alcohol to electric current must occur prior to a new cycle, the current being limited by the load resistance of the measurement circuit.
More recent instruments have utilized lower load resistance to shorten the time to reach the peak output and speed up the recovery time, and they integrate the output signal to obtain enhanced accuracy. However, the number of positive samples analyzed in rapid succession with these prior art instruments still had to be strictly limited. Successive readings might be in error as peak fuel cell output decreased because of the time required for the cell to complete the alcohol conversion reaction. This could conceivably give readings beyond the acceptable limits for evidential measurement. In a typical unit, ten successive measurements of 0.100 gm/dl gas at three minutes between readings might result in the tenth reading being 0.095 or 0.094. Accordingly, these instruments have unfortunately been limited to no more than five positive tests per hour for maintenance of evidential accuracy. Consequently, only one subject could be tested per hour with evidential accuracy in those jurisdictions requiring two tests per subject, and a third test if the first two differed by more than a given amount, and an additional a test reading on a standard to verify calibration of the instrument. In addition, once the fuel cell output of these instruments decreases due to repeated testing, an extended period of time (up to sixteen to twenty-four hours) is required before there is full recovery to the initial output.
Another problem with operating the fuel cell in the conventional mode with a load resistor is that although the output is very linear up to alcohol levels of about 0.150 gm/dl, the readings are increasingly low at higher levels. Thus the cell reads 2-3% low at 0.200 gm/dl, and 5-6% low at 0.300 gm/dl. This was of very little practical significance where legal maximum blood alcohol levels were fixed at 0.100 or 0.080, but it was a subject of criticism.
In early 1986, in a further investigation of fuel cell output measurement, based on a supposition that the entire signal from the fuel cell, rather than just the peak value, might be useful. The integrated output might contain enough information so that when the signal was analyzed properly, the effects of memory and high alcohol level nonlinearity might be minimized. Table 1 shows the results of a study made early in 1994. The investigators used a compressed gas standard with 0.100 ethanol concentration. The investigators made tests three minutes apart, at an ambient temperature of 23xc2x0 C. While the peak value varies by 13%, the calibrated fuel cell output integral remains constant.
It is also known to operate the fuel cell with the output essentially short-circuited, which gives the fastest response as shown in FIG. 2. With this configuration, cell output peaks in two to five seconds and typically returns to zero by the time that a cell with a 300-ohm load is reaching its peak value. In this mode, the fall-off in peak values from test to test is much worse than in the mode with a resistor; however, by integrating the entire area under the curve, the slump in reading from test to test is virtually eliminated. Because the cell has already returned to zero output, it is ready for another test without an additional waiting for a cleanup period to complete the reaction. The readings also recover much more quickly after a series of tests. For practical purposes, the number of tests per hour is limited by the recycling time of the test instrument and test protocols rather than the performance of the fuel cell. Research also established that a cell used in this mode is capable of linear response out to 0.400 gm/dl with an error no greater than 2%. For example, the linearity of a cell that was linear up to 0.150 gm/dl in a conventional voltage mode is preserved out to 0.400 gm/dl or more. A prior art circuit that provides an analog output in response to a fuel cell being loaded selectively with 330 ohms of resistance or essentially a short circuit is shown in FIG. 3, the short circuit being applied in response to an external signal.
Studies made by the Transportation Research Board concluded that blood alcohol concentrations (BAC) below 0.050% may impair driving-related skills. Further testing has shown that instruments using fuel cells showed greater accuracy at low BACs than the instruments using infrared techniques. In yet additional tests, investigators at the University of Tennessee at Memphis measured the response of fuel cell-based alcohol breath testing instruments to various substances including many that might be expected to be present in the breath of individuals being tested. In addition to separate responses to various non-alcoholic substances, ethanol, methanol, and isopropanol were separately introduced at alcohol concentrations of 0.1 gm/dl. The results indicate generally that the sensitivity to the non-alcoholic substances was from zero to about 2%; for ethanol the response was 100%; and for methanol and isoproponal the response was about 45%.
Notwithstanding the above developments, the breath alcohol instruments and interlock devices of the prior art are not entirely satisfactory, typically exhibiting one or more of the following disadvantages:
1. They are ineffective in distinguishing human breath samples, properly delivered, from non-human and/or improperly delivered samples; and
2. They are unreliable in that they are adversely affected by variations in ambient pressure (altitude) and temperature, as well as by the temperature and/or humidity of breath samples.
Although it is known to require a threshold dynamic pressure of the sample as well as to require a valid range of temperature and humidity of the sample, calibration of the instruments of the prior art to reject all non-human samples often results in unwanted rejection of proper human samples, and vice-versa. Also, even with breath samples properly accepted as human, there is still a likelihood of significant error in measured breath alcohol levels due variations in ambient conditions as well as the temperature and humidity of breath samples, even when the temperature and humidity are within prescribed limits and dynamic pressure maintained over a full required duration of time.
Thus there is a need for a breath alcohol interlock device that avoids the disadvantages of the prior art.
The present invention meets this need by providing a breath alcohol interlock device that is particularly effective in distinguishing permitted from disallowed alcohol levels, having quick response characteristics, and is both easy to operate and inexpensive to provide. In one aspect of the invention, a breath measurement instrument includes means for receiving a breath sample; and means for validating the sample, including means for determining a sample temperature of the sample, means for determining a moisture content of the sample, and means for comparing the determined sample temperature and moisture content with a predetermined profile of valid temperatures and moisture contents, validation being blocked unless the determined temperature and moisture content is within the predetermined profile, wherein the predetermined profile includes a valid range of the determined sample temperature that is dependent on the determined sample moisture content. The means for receiving the breath sample can include a tubular conduit having a mouthpiece extremity, the conduit defining a sample passage. The means for determining the sample temperature can include a temperature sensor supported relative to the tubular conduit and projecting into the sample passage, a sample temperature circuit having a sample temperature output, and means for signifying at least an out-of-limit temperature, and an in-limit temperature of the breath sample. The means for determining humidity can include a humidity sensor supported relative to the tubular conduit and projecting into the sample passage, a humidity circuit having a sample humidity output, and means for signifying at least an out-of-limit humidity and an in-limit humidity, the validation being blocked unless the temperature and humidity outputs are both in-limit. The in-limit humidity can be a first in-limit humidity, the humidity output also being capable of signifying a second in-limit humidity, the in-limit temperature also being a first in-limit temperature, the temperature output also being capable of signifying a second in-limit temperature, at least one combination of in-limit temperature and in-limit humidity being outside of the predetermined profile, that combination blocking the validation. In this way the range of acceptable sample temperatures is advantageously adjusted according the moisture content of the sample for facilitating effective distinguishing of non-human and/or improperly delivered breath samples from valid ones.
The invention also provides a breath alcohol instrument including the breath measurement instrument in combination with means for determining an alcohol content of the sample. The means for determining the alcohol content can include an alcohol-specific fuel cell, and further, a fuel cell circuit for producing a breath alcohol signal, and means for compensating the breath alcohol signal in response to variations in one or more variables of the set consisting of ambient temperature, ambient pressure, sample temperature, and sample humidity.
The invention further provides a breath alcohol interlock device for preventing use of a machine by an intoxicated operator, the device including the breath alcohol instrument in combination with an interlock circuit for disabling the machine except upon validation of a breath sample having an alcohol content below a predetermined amount.
In another aspect of the invention, a method for screening breath samples and determining an alcohol content thereof includes receiving a breath sample; validating the sample by determining a sample temperature of the sample, determining a moisture content of the sample, comparing the determined sample temperature and moisture content with a predetermined profile of valid temperatures and moisture contents, and blocking the validation unless the determined temperature and moisture content is within the predetermined profile, wherein the predetermined profile includes a valid range of the determined sample temperature that is dependent on the determined sample moisture content; determining an alcohol content of the sample by producing a breath alcohol signal responsive to the alcohol content of the sample; and compensating the breath alcohol signal in response to variations in one or more variables of the set consisting of ambient temperature, ambient pressure, sample temperature, and sample humidity.